1. Skin fibroblasts from patients with adrenoleukodystrophy (ALD) have elevated hexacosanoic acid in their complex lipids. The concentration thereof is substantially reduced on adding 20-50 MuM oleic acid to the medium. Oleic acid also reduces the C-26 fatty acid content in normal fibroblasts. Oleic acid was shown to inhibit synthesis of very long chain fatty acids (VLFA) from labeled acetate. The effect may indicate a mechanism for in vivo regulation of VLFA content in tissues and it could possibly be useful in treatment of patients with ALD. 2. An expanded series of fibroblast lines from 4 patients with Sjogren-Larsson Syndrome (SLS), a neurological disorder in which there are abnormally low levels of serum linolenic and arachidonic acids, was studied for Delta5- and Delta6-fatty acid desaturase activities. There was no significant difference between the activities in patient and control cells. It was shown in normal fibroblasts that the relative distribution of labeled arachidonic acid in various complex lipids is the same for the exogenous arachidonate taken up from medium and for that produced endogenously by desaturation and elongation of linoleic acid. 3. Samples of epidermal scales from 9 patients with several forms of hyperproliferative scaling disorders were analyzed for VLFA and long chain hydrocarbons. A relatively high content of both classes of compounds was found in some of the samples of epidermal scales.